Distance sampling for Sonoran Desert tortoises. Swann, D.; Averill-Murray, R.; and Schwalbe, C. Journal of Wildlife Management, 66(4):969–975, U.S. Geological Survey, Sonoran Desert Field Station, University of Arizona, Tucson, AZ 85721, United States, 2002.
abstract   bibtex   
We used line transects and distance sampling in combination with radiotelemetry to estimate density of a desert tortoise (Gopherus agassizii) population in the Rincon Mountains near Tucson, Arizona, USA, as part of a long-term study evaluating the impact of urban development on tortoises. During 2000, 34 1-km transects were each sampled twice in the 368.5-ha study area. We observed 46 tortoises with midline carapace lengths ?150 mm (subadults and adults) plus 7 juveniles on transects. For subadults and adults, the encounter rate was 0.63 tortoises/km, and the mean proportion of tortoises observable during radiotelemetry, conducted concurrently with transect sampling, was 82%. Corrected mean density based on line transects and radiotelemetry was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute abundance in the study area was estimated to be 193 (CV = 23.0%, CI = 107-291). Using the 2 independent coverages of transects as separate samples, the Lincoln-Petersen mark-recapture estimator produced an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%, CI = 72-440). Transects measured on the ground over uneven topography resulted in 3% smaller estimates of density when compared to analysis with transect lengths determined from coordinates plotted on a map. Distance sampling appears to be a feasible method of estimating density of Sonoran Desert populations of the desert tortoise, but transect lengths should be based on mapped rather than measured distances to prevent biases caused by uneven topography.
@ARTICLE{Swann2002,
  author = {Swann, D.E. and Averill-Murray, R.C. and Schwalbe, C.R.},
  title = {Distance sampling for Sonoran Desert tortoises},
  journal = {Journal of Wildlife Management},
  year = {2002},
  volume = {66},
  pages = {969--975},
  number = {4},
  abstract = {We used line transects and distance sampling in combination with radiotelemetry
	to estimate density of a desert tortoise \textit{(Gopherus agassizii)}
	population in the Rincon Mountains near Tucson, Arizona, USA, as
	part of a long-term study evaluating the impact of urban development
	on tortoises. During 2000, 34 1-km transects were each sampled twice
	in the 368.5-ha study area. We observed 46 tortoises with midline
	carapace lengths ?150 mm (subadults and adults) plus 7 juveniles
	on transects. For subadults and adults, the encounter rate was 0.63
	tortoises/km, and the mean proportion of tortoises observable during
	radiotelemetry, conducted concurrently with transect sampling, was
	82%. Corrected mean density based on line transects and radiotelemetry
	was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute
	abundance in the study area was estimated to be 193 (CV = 23.0%,
	CI = 107-291). Using the 2 independent coverages of transects as
	separate samples, the Lincoln-Petersen mark-recapture estimator produced
	an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%,
	CI = 72-440). Transects measured on the ground over uneven topography
	resulted in 3% smaller estimates of density when compared to analysis
	with transect lengths determined from coordinates plotted on a map.
	Distance sampling appears to be a feasible method of estimating density
	of Sonoran Desert populations of the desert tortoise, but transect
	lengths should be based on mapped rather than measured distances
	to prevent biases caused by uneven topography.},
  address = {U.S. Geological Survey, Sonoran Desert Field Station, University
	of Arizona, Tucson, AZ 85721, United States},
  file = {Swannetal2002.pdf:Swannetal2002.pdf:PDF},
  keywords = {Abundance estimation, Arizona, Density estimation, Desert tortoise,
	Distance sampling, Gopherus agassizii, Line transect, Radiotelemetry,
	Sonoran Desert},
  owner = {eric},
  subdatabase = {distance},
  timestamp = {2006.11.05}
}
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